Coating films of silicone resins can impart desired properties such as weather resistance, chemical resistance, impact resistance, heat resistance, freeze resistance and adhesion by a choice of a proper resin. The silicone resins are required to ensure formation of a coating by application, firm adhesion to a substrate, ease of working, and formation of ceramic films by firing at or above 400.degree. C. None of currently available silicone resins satisfy all the requirements.
Typical of the prior art is a composition comprising a resin of T units (R--SiO.sub.3/2) and a sol of Q units (SiO.sub.4/2).
This composition is used in forming a ceramic film since organic groups attached to silicon are less and a large amount of SiO.sub.2 is left after curing. For example, U.S. Pat. No. 4,027,073 discloses a hard coat composition comprising (i) colloidal silica and (ii) a partial condensate of Si--(OH).sub.3 units. This composition is improved in coating formation and substrate adhesion, but can form only a very thin film because of a low viscosity and low solids. If this coating is fired in order to form a ceramic film, the coating will crack or even disappear. Since organic groups attached to all the units of the resin burn upon firing, there occur volume shrinkage and gas evolution, by which the film can be cracked.
JP-B 49-39161 discloses a solvent-free liquid organic silicone resin in the form of a liquid silicone resin of T units serving as a ceramic precursor. This resin is improved in adaptation and adhesion to the substrate surface and formation of a thick film by firing, but from the working standpoint, it has a drawback that cross-linking prior to firing is necessary. Since the resin of the cross-linking cure type forms an integrated film by cross-linking reaction prior to firing, the film cannot accommodate the distortion caused by volume shrinkage upon firing and thus tends to crack.
Another silicone resin which can be fired is a MQ resin (MQR) in which the nucleus of a polycondensate of Q units is end-blocked with M units (R.sub.3 SiO.sub.1/2). Since the MQR has SiO.sub.2 at the center nucleus, it can be readily fired into ceramic. However, the MQR is a less agglomerative, less heat softening solid by itself and thus has a low film-forming capability so that the MQR is difficult to use as a coating material as such. Therefore, on use, the MQR is usually mixed with a diorganopolysiloxane and an organic solvent to form an elastomer-forming solution. Its well-known application is a pressure-sensitive adhesive (PSA).
One typical example of the silicone PSA is a composition comprising 40 to 80 parts by weight of MQR and 20 to 60 parts of a diorganopolysiloxane as disclosed in Japanese Patent No. 2686033. Since the coatability, adaptability and adhesion to the substrate surface and thick film formation are intended, this composition must contain a relatively large proportion of the diorganopolysiloxane. In the working examples, MQR and the diorganopolysiloxane are used in a weight ratio between 68/32 and 58/42. Compositions containing a higher proportion of MQR are not described. Since the PSA composition is designed to exert tack below the pyrolysis temperature, film formation by firing is not considered. If this composition is fired, the diorganopolysiloxane accounting for a relatively large proportion of the composition decomposes and volatilizes (see M. B. Neiman, De terioration of Polymers--Mechanism and Prevention--, Sangyo Tosho K. K., pp. 270-271 and 276), forming voids in the fired film. The resulting film is of low strength.
Furthermore, JP-A 7-310016 discloses a high solids silicone composition comprising (i) 50 to 90 parts by weight of MQR and (ii) 10 to 50 parts by weight of an alkenyl-containing diorganopolysiloxane. This composition may contain an organic solvent as an optional component. However, since the important advantage of this patent is the high solids, the amount of solvent is limited to 40 parts by weight or less per 100 parts by weight of the solids. No reference is made to the film formation, substrate adhesion and workability of a composition containing a more amount of the solvent. The weight ratio of MQR/diorganopolysiloxane is most preferably from 50/50 to 80/20. No compositions containing MQR in excess of 80/20 are prepared in the working examples. The only application described is addition curable PSA. An adhesion test is carried out in every working example. These apparently indicate that the composition was developed solely as a solventless or low solvent type PSA.